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Why Study Momentum Transport? Momentum transport is an important issue in: Accretion Disks Astrophysical Jets Solar Interior Laboratory Experiments Collisional viscosity fails to explain transport of momentum in all of the above cases Magnetic fluctuations can have a large, often dominant effect on the system in all of these situations A theme of Center research in this area is to significantly further our understanding of when and how magnetic fluctuations contribute to momentum transport

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Thin disk of material orbits a compact object and slowly falls onto it Angular momentum must be removed from accreting material: Leading explanation for this is torque associated with magnetic fluctuations Protostellar disk+jet (Hubble Space Telescope) Accretion Disks

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Internal Rotation Profile of the Sun Helioseismology shows the internal structure of the Sun. Surface differential rotation is maintained throughout the convection zone Solid body rotation in the radiative interior Thin matching zone of shear known as the tachocline at the base of the solar convection zone How does this come about? Momentum sources + transport

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Upgraded CHERS system installed on MST (April 2004) Initial measurements made on CVI line emission (~344 nm) Data exhibit large signal, low signal-to-noise Will allow impurity T i, v i to be resolved on fast time scale (~ 100 s) Atomic modeling & initial fitting of CVI line shape has been done time (ms) T i (eV) Beam ON Beam off

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Momentum Transport Physics and Plans 1. Momentum transport by stochastic magnetic fields 2. Momentum transport by Maxwell stress from current-driven instabilities 3. Momentum transport by Maxwell stress from magnetorotational instability 4. Generation and relaxation of momentum as part of a 2-fluid form of magnetic relaxation 5. Momentum transport in the sun

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Current-Driven Tearing Modes Perturbations with k·B = 0 do not bend B field lines Fluctuations with k·B = 0 somewhere are called resonant Position (surface) where k·B = 0 called resonant surface In MST, have helical B helical resonant perturbations Pitch of B field lines changes with radius Multiple resonances throughout plasma Tearing Modes One class of resonant perturbations Driven primarily by J(r) Tear magnetic field to form islands Typically see full spectrum of tearing modes in MST toroidal direction radius Puncture plot for single mode

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How far from ideal can the plasma be? –Some are quite resistive: protostellar disks, quiescent cataclysmic variables, etc. Can AMT be explained by hydrodynamic instabilities? Can MRI exist only when > 1 ? Do simulations get the transport rate right? Answer to latter two questions may be No if the scale height of the magnetic field is much larger than that of the plasma: a magnetized corona. Outstanding Issues Concerning MRI

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